1. Field of the Invention
The present invention generally relates to a multichip module and, more particularly, to a multichip module having LSI chips mounted on a wiring substrate of a composite structure.
In regard to a multichip module having LSI chips mounted on a wiring substrate, there are such requirements that the LSI chips be densely mounted, have high operating frequencies, be mounted in an arrangement that is easy to repair, and have a good heat radiation property.
In addition, there is a trend that LSI chips will have a micronized pattern and have narrowly pitched electrodes in the future. Accordingly, a wiring substrate on which these LSI chips are mounted needs to have a structure that can accommodate these LSI chips.
2. Description of the Related Art
A conventional chip module 10 comprises an LSI-chip-mounting wiring substrate 11, and an LSI chip 20 mounted thereon, as shown in FIG. 1. The LSI-chip-mounting wiring substrate 11 comprises a rigid substrate 12 formed of a glass epoxy and having a core material therein, built-up layers 13 and 14 formed on the upper and under surfaces of the rigid substrate 12, bumps 15 formed on the undersurface of the built-up layer 14, and an LSI chip-mounting part 16 provided on the upper surface of the built-up layer 13. The LSI chip 20 is mounted on the LSI-chip-mounting part 16 in a flip-chip form. The chip module 10 is mounted on a motherboard 30.
Since the chip module 10 employs a buildup method, the manufacture of the LSI-chip-mounting wiring substrate 11 does not exhibit a good yield. Additionally, due to a relatively low degree of evenness of the surface of the rigid substrate 12, an L/S (line/space) of a wiring pattern of the LSI-chip-mounting part 16 is limited to approximately 25/25 xcexcm not narrow enough to cope with electrodes of the LSI chip to be narrowly pitched.
Inventors of the present invention proposed a multichip module solving the above-mentioned problems. Japanese Laid-Open Patent Application No. 2000-353765 describes this multichip module. As shown in FIG. 2, this multichip module 40 comprises an LSI-chip-mounting wiring substrate 41, and LSI chips 45 and 46 mounted thereon in a flip-chip form. The multichip module 40 is mounted on the motherboard 30.
The LSI-chip-mounting wiring substrate 41 has a composite structure comprising a rigid substrate 42 formed of a glass epoxy and having a core material therein, and a flexible substrate 43 bonded on the upper surface of the rigid substrate 42. An LSI-chip-mounting part 44 is formed on the upper surface of the flexible substrate 43.
The flexible substrate 43 is manufactured by sputtering and etching on a surface of a glass board having a high degree of evenness, which are eventually removed from the glass board.
Since the LSI-chip-mounting wiring substrate 41 is manufactured by bonding the flexible substrate 43 manufactured independently of the rigid substrate 42 on the upper surface of the rigid substrate 42, the manufacture of the LSI-chip-mounting wiring substrate 41 exhibits an improved yield, compared with the chip module 10 employing the buildup method. Additionally, an L/S of a wiring pattern of the LSI-chip-mounting part 44 becomes smaller than approximately 25/25 xcexcm, narrow enough to cope with electrodes of the LSI chip to be narrowly pitched.
In the multichip module 40, however, the LSI chips 45 and 46 can be mounted only on the upper surface of the LSI-chip-mounting wiring substrate 41; thus, it is difficult to mount the LSI chips densely enough.
It is a general object of the present invention to provide an improved and useful multichip module in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a multichip module which can have LSI chips mounted densely enough on a wiring substrate.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a multichip module comprising:
a rigid substrate including a core material and having an opening;
a thin film fixed on an upper surface of the rigid substrate so as to be electrically connected to the rigid substrate and to close the opening, the thin film having an upper chip mounting part on an upper surface thereof and an under chip mounting part on an under surface thereof;
a first chip mounted on the upper chip mounting part; and
a second chip mounted on the under chip mounting part so as to be located in the opening,
wherein the rigid substrate and the thin film form a wiring substrate having a composite structure.
According to the present invention, the chips are mounted not only on the upper surface of the thin film but also on the under surface thereof. Thus, the chips are densely mounted on the wiring substrate.
The distance between the chip mounted on the upper surface of the thin film and the chip mounted on the under surface thereof becomes as extremely short as the thickness of the thin film. In a case where the upper and under chips are semiconductor chips, a delay of a signal transmitted between the upper and under semiconductor chips becomes so small that the delay can be ignored. This enables operating frequencies of the semiconductor chips to be increased. In another case where one of the chips is a semiconductor chip, and the other is a chip capacitor, inductance between the semiconductor chip and the chip capacitor becomes extremely small so as to make a full use of the chip capacitor; this enables elimination of high-frequency noises.
Additionally, in the multichip module according to the present invention, the thin film may comprise a first electrode pad formed on the upper chip mounting part, a second electrode pad formed on the under chip mounting part, and a via hole connecting the first electrode pad and the second electrode pad.
According to the present invention, bumps of the chip mounted on the upper chip mounting part of the thin film and bumps of the chip mounted on the under chip mounting part thereof are electrically connected to each other by way of the via hole formed in the thin film. This makes a delay of a signal transmitted therebetween so small that the delay can be ignored, and consequently, enables operating frequencies of the semiconductor chips to be increased. Thereby, the upper and under chips can function as one large-sized chip having functions of both the upper and under chips. Therefore, the cost of manufacturing the chips can be lowered while the performance of the chips is maintained.
Additionally, the multichip module according to the present invention may further comprise:
a lid joined to the upper surface of the rigid substrate so that the lid contacts and covers the first chip; and
a sealing member covering the second chip located in the opening.
According to the present invention, the lid and the sealing member protect the chips. Also, the lid encourages heat radiation so as to effectively cool the chip.
Additionally, in the multichip module according to the present invention, the first chip may produce a larger amount of heat than the second chip.
According to the present invention, the chip mounted on the upper surface of the thin film can be effectively cooled.
In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention a printed board unit comprising:
a printed board; and
a multichip module mounted on the printed board via at least one bump, the multichip module including:
a rigid substrate including a core material and having an opening, the bump being formed on an under surface of the rigid substrate;
a thin film fixed on an upper surface of the rigid substrate so as to be electrically connected to the rigid substrate and to close the opening, the thin film having an upper chip mounting part on an upper surface thereof and an under chip mounting part on an under surface thereof;
a first chip mounted on the upper chip mounting part; and
a second chip mounted on the under chip mounting part so as to be located in the opening,
wherein the rigid substrate and the thin film form a wiring substrate having a composite structure.
According to the present invention, the printed board unit can include densely mounted chips.
In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention a wiring substrate comprising:
a rigid substrate including a core material and having an opening; and
a thin film fixed on an upper surface of the rigid substrate so as to be electrically connected to the rigid substrate and to close the opening, the thin film having an upper chip mounting part on an upper surface thereof and an under chip mounting part on an under surface thereof,
wherein the rigid substrate and the thin film form a composite structure.
According to the present invention, since the opening is formed in the rigid substrate, and the thin film closes the opening, the chips can be mounted not only on the upper surface of the thin film but also on the under surface thereof.
Forming the chip mounting parts on the thin film can make an L/S smaller than forming the chip mounting parts on the rigid substrate. Thereby, a pitch between electrode pads becomes narrow so as to deal with narrowly pitched electrodes of LSI chips henceforth.
In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention a method of manufacturing a wiring substrate having a composite structure composed of a rigid substrate including a core material and having an opening, and a thin film fixed on an upper surface of the rigid substrate so as to be electrically connected to the rigid substrate and to close the opening, the thin film having an upper chip mounting part on an upper surface thereof and an under chip mounting part on an under surface thereof, the method comprising the steps of:
preparing a stage having a raised portion corresponding to the opening;
setting the rigid substrate on the stage by inserting the raised portion into the opening;
placing the thin film on the rigid substrate and an upper surface of the raised portion so that the thin film covers the opening; and
joining the thin film to the rigid substrate.
According to the present invention, the thin film is supported on the upper surface of the raised portion of the stage. This prevents the thin film from bending downward, even when the opening of the rigid substrate is large. Since, the thin film is supported horizontally, the manufacturing steps of the wiring substrate can be performed reliably.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.